1. Field of the Invention
This invention relates to voice and data communications, and more particularly to systems and methods to process received signals in communication systems.
2. Description of the Prior Art
There are several communication standards in commercial use. For example, the Institute of Electrical and Electronic Engineers (IEEE) has established a wireless standard, IEEE 802.16e. The IEEE 802.16e standard (IEEE 802.16e) outlines Media Access Control (MAC) and Physical Layer (PHY) specifications for wireless networks. The specification of the IEEE 802.16e addresses transmission of data in wireless networks. In particular, the IEEE 802.16e standard addresses communication in wireless asynchronous transfer mode (ATM) systems, covering frequencies of operation between 2.5 gigahertz (GHz) and 6 GHz. As is known in the art, IEEE 802.16e uses a modulation method called orthogonal frequency-division multiplexing access (OFDMA), which allows communication to occur at extremely high data speeds by transmitting data over multiple frequency channels over a wide frequency range.
The IEEE 802.16e specification includes mechanisms to maximize data transmission and reception reliability in packet transmission. Typically, several processes are performed in the receiver to successfully receive the transmitted data, including: synchronization, channel estimation and equalization, OFDM demodulation (e.g., by Fast Fourier Transforms), demapping, de-interleaving, decoding, and descrambling. The more relevant sections of the IEEE 802.16e specification applicable to the discussion below include sections 8.4.2, 8.4.4, and 8.4.6, which are hereby incorporated by reference.
The synchronization information of the received signals play a crucial role. For example, in OFDM systems, the frame and symbol boundary detection is very critical for reliable links between a transmitter and a receiver. One exemplary wireless communication network system is disclosed in the Mobile WiMAX Technical Overview and Performance Evaluation document prepared on behalf of the WiMAX Forum and published on Feb. 21, 2006, which is hereby incorporated by reference.
There are existing schemes for frame and symbol boundary detection. The synchronization information is typically determined by performing some operations, such as auto-correlation, cross-correlation, covariance, cross-variance of the received signals, or an equivalent. However, in the timing estimation methods presently used, the correlation calculations have high computational complexity and power consumption.
One paper that suggests the substantial challenges in synchronization in communication systems is entitled “Robust Frequency and Timing Synchronization for OFDM,” written by Timothy M. Schmidl and Donald C. Cox, and published in the IEEE Transactions on Communications, Vol. 45, No. 12, pp. 1613-1621, (December 1997), which is incorporated by reference. This paper discusses synchronization methods, particularly for orthogonal frequency-division multiplexing (OFDM).
A second paper that suggests the substantial challenges in synchronization in wireless communication systems is entitled “ML Estimation of Time and Frequency Offset in OFDM Systems,” written by Jan-Jaap van de Beek, Magnus Sandell, and Per Ola Borjesson, and published in the IEEE Transactions on Signal Processing, Vol. 45, No. 7, pp. 1800-1805, (July 1997), which is incorporated by reference. This paper discusses time offset and offset in carrier frequency estimations, particularly for OFDM.
FIG. 1 illustrates a flowchart of a method to generate synchronization information from received analog signals, according to the prior art. The sequence starts in operation 102. Operation 104 is next and includes inputting a received signal into an analog-to-digital (A/D) converter. Operation 106 is next and includes generating synchronization information from the output of the A/D converter. The method ends in operation 108.
FIG. 2 illustrates a receiving system that produces synchronization information, in accordance with the prior art. A/D converter 202 takes received analog signal 204 as an input and then produces a digital output 206 coupled to the synchronization information extraction module 208. Synchronization information extraction module 208 produces synchronization information 210 as an output. However, as previously noted, this module has high complexity and uses considerable power.
At higher frequencies, the signal is more directional and more easily interrupted by relative movements of the transmitter and/or receiver. Furthermore, at higher frequencies the amount of data transmitted in a unit of time increases, creating a need to avoid or minimize interruptions caused by synchronization failures. Therefore, a synchronization algorithm should be optimized as much as possible to deal with the greater vulnerabilities and consequences of the higher frequencies and faster data transmission environments.
It should be noted that some of the inventions of wireless communication systems are being adopted in wired communication systems (e.g., cable networks). Therefore, although the discussion above and below is directed to wireless communication applications, some communication challenges and solutions are common to both wireless and wired communication applications.
In view of the foregoing, what is needed is an improved method and system to generate synchronization information. Both wideband and narrowband wireless communication applications, and even wired communication applications, could benefit from such methods and systems.